Field of the Disclosure
The present invention relates to a cement nanoclay composition and a method for cementing hydrocarbon-producing wells, under high pressure and high temperature (HPHT) conditions, using the cement and nanoclay composition. The cement-nanoclay composition comprises water, hydraulic cement, nanoclay, admixed silica flour, optionally admixed with at least one additive selected from fluid loss agent, retarder, expanding agent, friction reducing agent, density reducing agents and weighting agents.
Description of Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present disclosure.
The increased demand for oil and gas has led to exploration of petroleum reserves into high pressure and high temperature (HPHT) zones in deeper formations. Drilling into HPHT zones requires a special design of cement slurry. Several types of additives are being investigated and used in high pressure and temperature wells to cater to the extreme environment.
Petroleum production and exploration strongly affects the global economic structure. Oil consumption increased by 171% during the period from 1965 to 2008. Over the last two decades, the amount of oil consumption per year has exceeded the amount of newly found oil reserves. With the continued growth of petroleum demand, oil and gas companies are exploring in new or unexplored areas. However, the search is proving to be extremely challenging in terms of depth, temperature and pressure. In deeper wells, high temperature and pressure and post-cementing operations put extreme stresses on the cement sheath and affect the integrity of the cement. In such conditions, the design of cement slurry is very critical and must have properties which ensures the durability and long term integrity of cement sheath.
In oil and gas wells after the well bore is completed, a pipe string is run into the well bore and the cement slurry is pumped into the annular space between the pipe casing and the formation rock in order to hold the pipe string in its place. This process is referred to as “primary cementing”. The fluid cement slurry hardens as the chemical reaction involving the formation of calcium silicate hydrate (CSH), C3S, C2S and C4AF takes place. The hardened cement sheath forms a layer separating the well bore formation and the casing, which is adhered firmly to the formation and the casing. The cement in the annular space holds the casing in place, and being highly impermeable, prevents the transport of corrosive fluid from the formation to the casing, thereby precluding the corrosion of the pipe string. It also provides a barrier which inhibits the migration of gases in the micro annulus between the formation and the cement and the cement and pipe casing.
Cementing in HPHT wells is complicated due to wide ranging temperature and pressure variations and stresses to which the annular cement sheath, between the casing and the formation, is subjected during its service life. The long-term integrity and durability of the annular cement depends on providing casing support and preventing the migration of formation fluid in liquid or gaseous form through or at the boundaries of the cement sheath. The zonal isolation requires a robust cement slurry design which provides a strong and durable cement-casing and cement-formation bonding, precludes bulk shrinkage by inhibiting the fluid loss, has zero free water settling of cement, and forms microannulus due to stress imbalance at the interface resulting from thermal regimes, hydraulic pressure or mechanical stresses. The hardened cement slurry should also resist radial fracturing which may result from shrinkage stresses, thermal expansion or contraction of the steel casing and pressure fluctuation, mechanical impact or other conditions within the casing. The HPHT wells have a larger probability of migration of gas and corrosive fluid and leakage. Therefore, special attention must be paid to cementing processes, especially in HPHT wells. Studies have shown that approximately 80% of the wells in the Gulf of Mexico have gas transmitted to the surface through the cement casing.
The appropriate cement slurry design for well cementing is a function of various parameters, including the well bore geometry, casing hardware, formation integrity, drilling mud characteristics, presence of spacers and washers, and mixing conditions. Communications between zones, gas migration, undesired fluid entry, strength retrogression and stresses are examples of the serious consequences resulting from poor cementing jobs in HPHT wells.
In order to ensure that the well safely produces hydrocarbons over its service life, it is necessary to ensure long-term durability of the cement composition. The cement sheath is subjected to large variations in thermal regime, stresses are generated in the cement sheath from work over activities in the well, pressure testing, production and other mechanical loadings.
The compressive and tensile strength of the cement matrix are generally considered to be indicative of the cracking in the cement and its propagation. When the tensile stress in the cement matrix exceeds the tensile strength, which is itself evolving with time, cracking will take place in the cement. Toughness of the cement matrix is an important material parameter governing the initiation and propagation of the cracks in the cement.
Cementing of HPHT wells using hydraulic cements is not feasible due to retrogression in compressive strength of the cement at temperatures exceeding 230° F. The hydrated lime released in the set cement may form alpha dicalcium silicate hydrate which results in strength retrogression of the cement. The hydrated lime may also leach out of the cement sheath resulting in deterioration of the cement matrix, and thereby enhancing the permeability which paves the way for the transport of gasses and corrosive fluids.
Permeability of the cement matrix is the key parameter which is indicative of the potential of gas migration and fluid transport in the cement. The cement in the annular space of well bore and casing undergoes a transition from a fluid phase to solid phase. It is important that the permeability of the cement during this transition and after it has achieved its full strength remains low to prevent the transport of formation fluids through the pores of the cement.
Recently, nanomaterials have demonstrated effectiveness across a variety of industries, from textiles and defense to aerospace and energy. They are now being used as commercially feasible solutions to technical challenges faced by many industries. Nanomaterials have high surface area and small size leading to beneficial properties which provides an impetus for its usage in oil and gas industry. Though nanotechnology has shown its presence in other industries throughout a few decades, its application in the oil and gas industry remains to be fully explored (Singh & Ahmed, 2010).
Development of high performance materials for construction is possible by utilizing the potential of nanotechnology. Nano-materials (being smaller in size and higher in surface area) are used in several fields, including catalysis, polymers, electronics, and bio-medical applications (Park & Road, 2004). Because of a higher surface area, these materials can also be used in oil/gas well cementing to accelerate the cement hydration process (Heinold, Dillenbeck, 2002). Due to their wide range of applications, they can help enhance final compressive strength and reduce fluid loss (Li & Wang, 2006; Campillo et al., 2007). Few literature reports are available mentioning nanomaterials in the concrete industry. For example, Campillo et al., (2007) investigated the effect of nano-alumina in belite cement. The study found that addition of nano-alumina enhances mechanical properties to some extent. Li et al., (2006) reported use of nano-SiO2 or nano-Fe2O3 in cement mortar. The results showed improvement in compressive and flexural strength compared to plain cement mortar. Patil & Deshpande (2012), Senff et al. (2010) and Ershadi et al. (2011) have reported that addition of nanomaterial such as nanosilica also results in a significant increase in the compressive strength of the cement mix and prevents strength retrogression at high temperature.
Some of the examples of harnessing nanotechnology in drilling fluids (Singh & Ahmed, 2010) suggest that nanotechnology can bring revolutionary changes to additive development.
The present disclosure demonstrates that a type of nanomaterial, referred to as nanoclay, helps improve the properties of cement in oil/gas wells subjected to HPHT conditions. A well located in Saudi Arabia was selected to study the cement mixture design. Nanoclay material was added at various percentages to the Saudi Type-G Cement and the beneficial impact of nanoclay on the strength, rheological and durability properties of the cement slurry was demonstrated.